1. Field of the Invention
The present invention relates to a lead-free and arsenic-free and preferably fluorine-free niobium phosphate optical glass, to the use of such a glass in the fields of mapping, projection, telecommunication, optical communication engineering, mobile drive and laser technology, as well as to optical elements and so-called “preforms” of such optical elements, respectively.
2. Related Art
In recent years the trend in the market in the fields of optical and opto-electronic technologies (application fields; mapping, projection, telecommunication, optical communication engineering, mobile drive and laser technology) is toward more and more miniaturization. This is manifested in smaller and smaller finished products and of course demands increasing miniaturization of the individual structural members and components of such finished products. For the producers of optical glass this development translates into a definite decrease of required raw glass volumes in spite of increasing quantities of finished products. At the same time increased pricing pressure on the glass manufacturers arises from the re-processors, since the production of such smaller components made of block and/or ingot glass produces a noticeably greater percentage of waste. Also a higher operating expense is required for processing such miniaturized parts than for bigger components.
Instead of removing glass portions for optical components from a glass block or ingot, which is common up to now, recently production procedures have become important, in which preforms, which are as close as possible to the final contour or final geometry, such as e.g. gobs or spheres, may be obtained immediately after the glass melt. For example, the re-processors' requests for preforms, which are close to the final geometry for re-pressing, so-called “precision gobs”, are increasing. Normally, the term “precision gobs” means preferably completely fire-polished, free or half-free formed glass portions, which are already portioned and have geometry, which is close to the final form of the optical component.
Such “precision gobs” may also preferably be converted into optical elements, which are lenses, aspherical elements, etc., by so-called “precise pressing” or by “precision molding” or “precision pressing”. (German expression: “Blankpressen”). These terms are synonymous. Then further processing of the geometric shape of the surface, e.g. with a surface polish, is no longer required. Because of this process the smaller volumes of melted glass (distributed in a large number of small parts of the material) are accompanied in a flexible way by shorter setting times. However, because of the comparatively lower cycle number or number of parts and due to the small geometry as a rule, the added value of the process cannot be based on the value of the material alone. Rather, the products must leave the press in a state ready for installation, i.e. laborious post-processing, cooling and/or cold re-processing must not be necessary. Because of the required high accuracy of geometries, high-grade precision instruments and therefore expensive mold materials have to be used for such a pressing procedure. The lifetimes of such molds greatly affect the profitability of the products and/or materials produced. A very important factor for a long life-time of the molds is a working temperature, which is as low as possible, but which can only be lowered to a point at which the viscosity of the materials to be pressed is still sufficient for the pressing procedure. This means, that there is a direct causal relationship between the processing temperature, and thereby between the transformation temperature Tg of a glass to be processed, and the profitability of this pressing process: The lower the transformation temperature of the glass, the longer the lifetimes of the molds; and therefore the higher the earnings. Thus, there is a demand for so-called “low Tg glasses”, i.e. glasses having low melting points and transformation temperatures, i.e. glasses with a viscosity which is sufficient for processing at temperatures, which are as low as possible.
Furthermore, from a procedural point of view of the melt, recently there is a growing demand for “short” glasses, i.e. glasses having a viscosity, which varies strongly within a certain viscosity range when there is a relatively small change in temperature. This viscosity behavior has the advantage that the times of hot forming in the melting process, i.e. the closure times of the molds, can be decreased. Because of that, on the one hand the throughput will be increased, i.e. the cycle times will be reduced. On the other hand, because of that also the mold material will be protected, which also has a positive effect on the total production costs, as described above. Such “short” glasses have the further advantage that also glasses with higher tendency to crystallize may be processed by faster cooling than with corresponding “longer” glasses. Therewith pre-nucleation, which could cause problems in succeeding steps of secondary hot forming, will be avoided. This makes it possible to draw such glasses to form glass fibers.
Furthermore it is also desirable that, besides the above-mentioned and the required optical properties, the glasses are sufficiently chemically resistant and have thermal expansion coefficients, which are as low as possible.
The prior art already describes glasses with a similar optical state or with a comparable chemical composition, but these glasses have huge disadvantages. In particular, many of the glasses contain higher proportions of SiO2, which is a network forming agent and therefore increases the transformation temperature of the glass, causes a longer viscosity curve and reduces the refractive index and/or the amounts of components, such as B2O3, Na2O and F, which readily can evaporate during the melting and burning process. Thus an exact adjustment of the glass composition is difficult. This evaporation is also disadvantageous during the pressing process, in which the glass is heated again and may deposit on the surface of the mold and on the glass.
According to the prior art larger amounts of the component titanium oxide (more than 4% by weight) are often used, however the tendency to crystallize will be increased undesirably and further the UV cut-off is shifted to longer wavelengths.
EP 1 078 894 discloses an optical glass for precision forming with a refractive index of at least 1.83 and an Abbe number of at most 26. In every case the glass contains Na2O in an amount of at least 2.5% by weight, which is a disadvantage because of the aforesaid volatility of this component.
JP 01219036 describes an optical glass with high refractive index and high dispersion. The glass contains in every case SiO2, which is a network-forming agent, in an amount of at least 5% by weight.
JP 2002173336 comprises a highly refractive optical glass with a refractive index of 1.75 up to 2.0 for precise pressing technology. The glass contains in every case 0.2 Mol % B2O3, which is volatile.
JP 09188540 describes a niobium phosphate optical glass having an improved stability to solarization. However it contains WO3 in a maximum amount of only 10% by weight. In combination with the other required components an advantageous refractive index of >1.86 cannot be achieved in this glass.
JP 06345481 describes the production of a P2O5—TiO2 glass with improved transmission. It contains TiO2 in a proportion of at least 5% by weight. Such a high content of TiO2 shifts the UV cut-off to longer wavelengths, which is not desired, and promotes devitrification of the glass.
JP 05-270853 describes a niobium phosphate glass with improved transmission and stability to devitrification, a refractive index of 1.53 to 1.85 and an Abbe number of 18 to 48. Nevertheless, it contains WO3 in a maximum amount of only 10% by weight. In combination with the other required components an advantageous refractive index of >1.86 cannot be thereby achieved.
JP 2002293572 describes optical glass for eyeglass lenses, which contains in every case the components B2O3 and Na2O. In addition, the P2O5 content is more than 32% by weight, which is so high that in combination with the other required components an advantageous refractive index of >1.86 cannot be thereby achieved.
JP 2003160355 describes an optical glass with a refractive index of higher than 1.83 for precision pressing. However the glass contains in every case the easily evaporated component Na2O.
JP 2001066425 comprises a substrate glass for optical filters with a thermal expansion coefficient of 9 to 12*10−6/K in the temperature range of −20 to +70° C. Glasses of the present invention generally have thermal expansion coefficients that are less than the thermal expansion coefficients of the glass of this JP reference, which is advantageous and which imparts the glasses with positive properties, so that they are insensitive to differences in temperature. Besides, according to this prior art reference a sum of the content of silicon, barium and phosphorous oxide of 35 to 55% by weight is desirable. With such a high content of these components in connection with the other required components an advantageous refractive index of >1.86 cannot be achieved.
EP 1 350 770 describes an optical glass with a refractive index of 1.88 and an Abbe number of 22 to 28. Nevertheless, it contains in every case at least 15% by weight of SiO2 and at least 5% by weight of TiO2.
JP 081004537 describes a highly refractive and high dispersion optical glass. Nevertheless, it contains in every case at least 1% by weight of B2O3.
JP 62128946 concerns a highly refractive telluric glass, which comprises toxic tellurium oxide as a component, in every case.
The documents JP 63170247, DE 4025814 and US Published Patent Application 2004/053768 describe optical glasses, which indeed can be free of lead and free of fluorine, however they contain SiO2 in every case.
The documents JP 2003238197, US Published Patent Application 2004/018933 and EP1468974 disclose optical glasses, which contain sodium oxide as a component, in every case.
JP 61040839 describes an optical phosphate glass, which contains at least 1% by weight Sb2O3 in every case.
EP 1 493 720 describes an optical glass for precise pressing. But this glass has an undesirably large thermal expansion coefficient of 11-18.4*10−6/K in the temperature range of 100 to 300° C. The glass comprises Li2O in an amount of more than 3% by weight.
US Published Patent Application 2005/0164862 discloses glass, which can contain either bismuth or tungsten oxide as a component, and also contains antimony oxide in the glass, in every case.
US Published Patent Application 2005/0159290 discloses glasses suited for precision molding comprising niobium oxide in an amount of less than 22% by weight. According to this application if amounts of niobium oxide are above 22% by weight an undesirable coloration of the glass occurs when it is exposed to UV light.
US 2005/0192174, which was published after the priority date of the present invention, describes glasses suited for precision molding comprising more than 14% by weight of germanium oxide. It is stated that the desirable refractive index cannot be achieved when less than 14% by weight of this compound are present in the glass.